Implement PlaceholderFormatter for template interpolation

I wanted to create a way to build dynamic, context-aware messages, similar to
what we currently have in the Validation component. My long-term goal is to
decouple string formatting from Validation and centralize it here in
StringFormatter. This commit introduces the `PlaceholderFormatter`, which
brings a familiar `{{placeholder}}` syntax to our toolkit.

Recognizing that creating a new object for every unique template can be
cumbersome, I have introduced the `formatWith()` method. This allows for
additional data injection at call-time. However, to maintain predictability,
I’ve established a clear hierarchy: parameters provided in the constructor are
treated as the "primary" configuration and will always take precedence over
call-time parameters. This ensures that a pre-configured formatter behaves
consistently even when used in a dynamic context.

By moving this logic here, we lay the groundwork for a more modular system
where formatting is a first-class citizen rather than a utility hidden
inside other components.

Assisted-by: Claude Code (Opus 4.5)

Author: henriquemoody

README.md

@@ -16,10 +16,11 @@ composer require respect/string-formatter
 
 ## Formatters
 
-| Formatter                                    | Description                                      |
-| -------------------------------------------- | ------------------------------------------------ |
-| [MaskFormatter](docs/MaskFormatter.md)       | Range-based string masking with Unicode support  |
-| [PatternFormatter](docs/PatternFormatter.md) | Pattern-based string filtering with placeholders |
+| Formatter                                            | Description                                         |
+| ---------------------------------------------------- | --------------------------------------------------- |
+| [MaskFormatter](docs/MaskFormatter.md)               | Range-based string masking with Unicode support     |
+| [PatternFormatter](docs/PatternFormatter.md)         | Pattern-based string filtering with placeholders    |
+| [PlaceholderFormatter](docs/PlaceholderFormatter.md) | Template interpolation with placeholder replacement |
 
 ## Contributing
 

composer.json

@@ -4,7 +4,8 @@
     "description": "A powerful and flexible way of formatting and transforming strings",
     "require": {
         "symfony/polyfill-mbstring": "^1.33",
-        "php": "^8.5"
+        "php": "^8.5",
+        "respect/stringifier": "^3.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^12.5",